Semiconductor-based single photon detectors (SPDs) are currently used in a wide range of areas, such as single molecule detection, fluorescent spectroscopy, optical fiber testing, basic quantum mechanics, and quantum information. These SPDs usually detect single photons via an electrical amplification process (also referred to as electrical avalanche process). In this process, a single photon first causes an atomic excitation, such as the generation of free electrons and holes, upon absorption by the semiconductor material in the SPD. The atomic excitations are then amplified through an electronic avalanche process. The amplified atomic excitations are then recorded as an electrical signal using thresholding and sampling mechanisms, followed by an analog-to-digital converter.
For example, in a state-of-the-art infrared (IR) Ge—Si avalanche photodetector (APD), a photon absorbed in the Ge region produces an electron-hole pair. The electron is accelerated under a bias voltage and tunnels into a Si avalanche region, where the electron is amplified above the electronic background noise. A thresholding mechanism then records the rising edge of the resulting electron pulse to determine the presence of the photon absorbed by the Ge region.
Single photon APDs can be constructed from several materials, such as germanium, GaAs, GaP, and GaAsP. However, the performance of these detector are usually limited by the optical absorption and electrical readout mechanisms. For example, jitter in the detectors can be caused or exacerbated by factors including the inhomogeneous travel time of carriers across the junction and the limited electrical readout bandwidth. Electronic defects in the amplification region can contribute to dark counts (also referred to as dark current), which can be particularly severe in non-Si APDs. In addition, despite decades of development of passive and active reset mechanisms, reset times are still on the order of hundreds of nanoseconds, which means that it takes at least hundreds of nanoseconds for a semiconductor-based SPD to detect a second photon after detecting the first one.